1. Field of the Invention
This invention relates generally to a method of processing metal, particularly suitable to process high melting point metal containing at least one of W and Ti, and to a method of manufacturing a semiconductor device using such metal.
2. Description of the Related Art
A thin film resistor made of CrSi or the like is formed on a semiconductor substrate to provide a desired value of resistance. This thin film resistor adopts, as an electrode structure, a structure of thin film resistor/barrier metal/Al wiring member in which the barrier metal made of high melting point metal containing at least one of W and Ti is sandwiched between the thin film resistor and the Al wiring member so as to prevent decrease in allowable current due to mutual diffusion between the thin film resistor and the Al wiring member.
FIGS. 9A and 9B show steps in a process for manufacturing a semiconductor device having a thin film resistor. FIG. 9A shows a state obtained through several steps, in which a thin film resistor J3 is patterned on an insulating film (oxide film) J2 provided on a silicon layer J1a of a SOI (Silicon On Insulator) wafer J1, and a barrier metal J4 is patterned on the thin film resistor J3. Al wiring members J5 are further formed on both ends of a stack of the thin film resistor J3 and the barrier metal J4, and on the oxide film J2. Photoresist J6 is deposited with an opening exposing the central portion of the barrier metal J4.
Next, as shown in FIG. 9B, the barrier metal J4 is etched to expose the thin film resistor J3. After that, although it is not shown, an interlayer insulating film and an Al thin film layer are formed and patterned, and then a protective film is formed. As a result, the semiconductor device is completed.
The etching of the barrier metal J4 shown in FIG. 9B is generally wet etching that uses, as an etching solution, hydrogen peroxide water (H2O2/H2O solution) or a solution containing hydrogen peroxide water mixed with alkali such as ammonia (H2O2/NH4OH solution). Here, H2O2/NH4OH solution is more generally used because it allows stable etching even when the barrier metal J4 has a somewhat damaged layer in a surface portion thereof and it contains no metallic ion that affects characteristics of a transistor formed in the semiconductor device.
However, if the surface of the thin film resistor J3 is exposed to H2O2/NH4OH solution during the etching, local current paths are formed between the thin film resistor J3 and the barrier metal J4 to cause a cell reaction locally. This is because the ionization tendency of the thin film resistor J3 is different from that of the barrier metal J4 and H2O2/NH4OH solution exhibits high electrical conductivity.
If once the cell reaction occurs, the etching rapidly progresses in the lateral direction of the barrier metal J4. This prevents the processing dimension (dimension between both barrier metal end portions) from being determined stably. Further, as indicated by arrows A in FIG. 9B, the barrier metal J4 may be excessively etched (over-etched) in the lateral direction at the portions under the Al wiring members J5. In this case, the interlayer insulating layer is difficult to fill the over-etched portions of the barrier metal J4, and it may produce vacancies in the over-etched portions. If the vacancies exist, the semiconductor device would have deficiencies such as water invading into the vacancies.
On the other hand, when H2O2/H2O solution containing no alkali is used, because its electrical conductivity is smaller than that of H2O2/NH4OH solution, etching is prevented from rapidly progressing. However, if the surface portion of the barrier metal J4 is damaged and oxidized during the manufacturing process to have a transformed layer, the etching using H2O2/H2O solution is practically difficult.
The present invention has been made in view of the above problems. An object of the present invention is to provide a method for processing a metal with high accuracy in case where different metals contact each other, and particularly a method for processing high melting point metal containing at least one of W and Ti with high accuracy.
According to the present invention, when a second metal overlying a first metal is wet-etched to expose the first metal, the wet etching is performed at first and second etching steps respectively using a first etching solution having a first electrical conductivity and a second etching solution having a second electrical conductivity smaller than the first electrical conductivity. The first etching step is performed using the first etching solution before the second etching step is performed using the second etching solution.
That is, the second etching solution having the smaller electrical conductivity is used when the first metal is exposed during the wet etching. As a result, a cell reaction can be prevented from occurring between the first metal and the second metal, and the etching can be prevented from rapidly progressing. The second metal can be processed (etched) securely with high accuracy.